1. Field of the Invention
The present invention relates to a method for producing a semiconductor device, and in particular to a method for producing a semiconductor device utilizing control of a specific resistance value of a semiconductor film through oxidation treatment or heat treatment.
2. Description of the Related Art
In recent years, various types of flexible devices are receiving attention along with appearance of oxide semiconductors, such as ZnO semiconductors. The flexible devices have various uses, such as electronic paper and flexible displays. However, for the oxide semiconductors, a resistance reduction (high-density carrier generation) technique, such as doping (impurity implantation) used for Si semiconductors, has not been established. Therefore, it is difficult to produce a semiconductor device including an active layer formed of the oxide semiconductor film and having desired properties. In order to address this problem, various techniques have been used to control the specific resistance value of the oxide semiconductor film by applying a post treatment to improve properties of the semiconductor device. For example, Japanese Unexamined Patent Publication No. 2007-142196 teaches to apply heat treatment at 500° C. to the oxide semiconductor film.
With the technique disclosed in the above patent document, however, it is impossible to produce a semiconductor device using a plastic substrate, for example. This is because that the structure of a flexible device basically includes a crystalline semiconductor or a metal film patterned on a flexible substrate, such as a resin substrate. Since the flexible substrates have a lower heat resistance than that of inorganic substrates, such as glass substrates, it is necessary to carry out the entire production process of the flexible device at a temperature not higher than the operating temperature limit of the substrate. The operating temperature limit of a resin substrate, for example, is usually 150-200° C., although depending on the material. Even a material having a relatively high heat resistance, such as polyimide, has the operating temperature limit of at most about 300° C.
Therefore, as the above-mentioned post treatment, use of laser annealing has been considered. In the laser annealing, high energy pulsed light, such as an excimer laser, is applied to a sample to instantaneously fuse and crystallize a semiconductor material. Thus, the specific resistance value of the oxide semiconductor film can be controlled without affecting the substrate.
In a case of a ZnO film, however, if the post treatment by laser annealing using the excimer laser is applied, the resulting ZnO film has excessively low resistance, and it is impossible to use the ZnO film as the active layer of the semiconductor device.